The present invention relates to the hydrogenation of a nitrile function of a dinitrile in order to obtain the corresponding aminonitrile.
Generally, the hydrogenation of dinitriles is carried out in order to prepare the corresponding diamines; thus, in particular, the hydrogenation of adiponitrile gives hexamethylenediamine, which is one of the two base compounds in the preparation of nylon-6,6.
However, it occasionally proves to be necessary to prepare not the diamine, but the intermediate aminonitrile. This is the case, for example, but not exclusively, for the hemihydrogenation of adiponitrile into aminocapronitrile, a compound which can then be converted into caprolactam, which is a base material for nylon-6, or directly into nylon-6.
Thus, U.S. Pat. No. 4,389,348 describes a process for the hydrogenation of dinitrile into xcfx89-aminonitrile, with hydrogen, in an aprotic solvent medium and ammonia and in the presence of rhodium deposited on a basic support.
U.S. Pat. No. 5,151,543 describes a process for the partial hydrogenation of dinitriles into aminonitriles in a solvent in a molar excess of at least 2/1 relative to the dinitrile, comprising liquid ammonia or an alkanol containing an inorganic base which is soluble in the said alkanol, in the presence of a catalyst of Raney nickel or cobalt type.
Patent application WO-A-96/18603 describes a process for the hemihydrogenation of aliphatic dinitriles into the corresponding aminonitriles, using hydrogen and in the presence of a catalyst chosen from Raney nickel and Raney cobalt, the said Raney nickel or cobalt preferably comprising a doping element such as chromium, iron, titanium or zinc, and a strong inorganic base derived from an alkali metal or alkaline-earth metal, the initial hydrogenation medium comprising water in a proportion of at least 0.5% by weight relative to the total amount of liquid compounds in the said medium, diamine and/or aminonitrile which are capable of being formed from the dinitrile to be hydrogenated, as well as unconverted dinitrile in a proportion for these three compounds together of from 80% to 99.5% by weight relative to the total amount of liquid compounds in the said medium.
As emerges from the preceding analysis, processes for the hemihydrogenation of dinitriles into aminonitriles usually use a catalyst of Raney nickel or cobalt type, preferably doped by the presence of one or more other elements.
It has now been found that the presence of copper and/or silver and/or gold in Raney nickel or cobalt gives, unexpectedly, excellent results in terms of selectivity towards aminonitrile.
The selective hydrogenation of a single nitrile function of a dinitrile is by nature very difficult to achieve, since the aminonitrile is the intermediate compound in the complete hydrogenation to diamine. This intermediate thus enters into competition with the starting dinitrile as a compound capable of being hydrogenated. Consequently, the overall distribution between aminonitrile and diamine obtained results from the ratio of the kinetic coefficients of hydrogenation of the first nitrile function (k1) and of the second nitrile function (k2).
It has been found that the use of a Raney cobalt or nickel doped with at least one metal chosen from copper, silver and gold makes it possible to improve the k1/k2 ratio of these kinetic coefficients and thus the selectivity of the hydrogenation towards aminonitrile relative to the complete hydrogenation into diamine, in comparison with the use of a Raney catalyst doped with the elements most commonly used, such as, for example, chromium, iron or titanium.
The present invention relates to the preferential hydrogenation of only one nitrile function of a dinitrile (also referred to in the present text as hemihydrogenation) so as to prepare the corresponding aminonitrile predominantly and the diamine only to a minor extent.
More specifically, the invention relates to a process for the hemihydrogenation of a dinitrile into the corresponding aminonitrile, in a liquid medium, characterized in that the process is performed in the presence of a Raney nickel or cobalt catalyst containing at least one metal chosen from copper, silver and gold and in the presence of an alkali metal or alkaline-earth metal hydroxide.
Raney nickels are catalysts that are widely used in the industry for hydrogenation reactions. They are prepared by alkaline attack of aluminium-rich Al/Ni or Al/Co alloys possibly containing other metals, which are generally referred to as doping agents or promoters. The catalyst consists of aggregations of nickel or cobalt crystallites with a large specific surface area and a variable residual aluminium concentration.
This catalyst generally comprises an aluminium content, expressed by weight relative to the weight of the nickel or cobalt, of less than or equal to 10%.
Advantageously, the (Cu+Ag+Au)/Ni or (Cu+Ag+Au)/Co weight ratio of the catalyst used in the present invention is between 0.05% and 10% and preferably between 0.1% and 5%.
Along with copper and/or silver and/or gold, the catalyst used in the process can contain amounts, generally smaller amounts, of one or more other elements also included under the generic term of doping agent or promoter.
These additional doping-agents which may be present are preferably chosen from the following elements: titanium, chromium, iron, zirconium, vanadium, manganese, bismuth, tantalum, rhodium, ruthenium, iridium, platinum, palladium, niobium, hafnium and rare-earth elements. When the catalyst used is Raney nickel, cobalt can also be present as an additional doping agent.
Similarly, when the catalyst used is Raney cobalt, nickel can also be present as an additional doping agent. Titanium, chromium, iron and zirconium are particularly preferred.
The amount of doping agent other than copper, silver and gold which the catalyst can contain generally represents from 0% to 5% by weight relative to the weight of the nickel or cobalt.
Among the catalysts of Raney type used, Raney nickel as defined above will be preferred.
The catalyst used in the present invention can also be used in the form of grains.
The process of the invention applies more particularly, but not exclusively, to the dinitrile substrates of formula (I):
NCxe2x80x94Rxe2x80x94CNxe2x80x83xe2x80x83(I)
in which R represents a linear or branched alkylene group containing from 1 to 12 carbon atoms.
Preferably, in the process of the invention, dinitriles of formula (I) are used in which R represents a linear or branched alkylene radical containing from 1 to 6 carbon atoms.
Examples of such dinitriles which may be mentioned in particular are adiponitrile, methylglutaronitrile, ethylsuccinonitrile, malononitrile, succinonitrile and glutaronitrile as well as mixtures of several of these dinitriles, in particular the adiponitrile, methylglutaronitrile and ethylsuccinonitrile mixtures derived from the same process for the synthesis of adiponitrile.
The concentration of dinitriles, in particular of adiponitrile, in the reaction medium can vary widely depending on the embodiment of the invention, continuous or batchwise operation, the initial load or gradual introduction, for example. In the preferred context of an industrial process operating continuously, the average dinitrile concentration is usually between 10% and 45% on a weight for weight basis.
The alkali metal hydroxides, LiOH, NaOH, KOH, RbOH and CsOH, and mixtures thereof, are preferably used.
In practice, NaOH and KOH are preferably used for a good performance-price compromise.
The hydrogenation reaction medium is liquid. It contains at least one solvent capable of at least partially dissolving the dinitrile substrate to be hydrogenated.
According to one advantageous embodiment of the process according to the invention, a liquid reaction medium which is at least partially aqueous is used. The water generally represents an amount of from 1% to 25% by weight relative to the total weight of the reaction medium. Preferably, the water content of the reaction medium is between 1 and 15% on a weight for weight basis.
At least one other solvent, generally of alcohol type, may be provided in addition to or in replacement for the water. The alcohols which are more particularly suitable are, for example, methanol, ethanol, propanol, isopropanol and butanol, and mixtures of the said compounds.
When it is used with water, the alcoholic solvent represents from two to four parts by weight per one part by weight of water and preferably three parts per one part of water.
According to another preferred characteristic of the invention, the initial hydrogenation medium comprises, in particular in the context of a continuous implementation of the process, diamine which is co-produced by the hydrogenation. An example is hexamethylenediamine, when the dinitrile substrate is adiponitrile.
The average concentration of aminonitrile and/or of diamine in the reaction medium under continuous operating conditions is advantageously between 35% and 90% by weight relative to the total weight of the solvent included in the said reaction medium, and is more preferably between 45% and 89% on a weight for weight basis.
The reaction medium can comprise liquid or dissolved ammonia. The ammonia generally represents from 0% to 50% of the weight of the reaction medium and preferably from 0% to 15%.
The amount of alkali metal or alkaline-earth metal hydroxide in the reaction medium varies depending on the nature of the said reaction medium.
Once the reaction medium contains only water, the reaction products and optionally ammonia, as liquid solvent medium, the amount of alkali metal or alkaline-earth metal hydroxide is advantageously greater than or equal to 0.1 mol/kg of catalyst, preferably between 0.1 and 2 mol/kg of catalyst and even more preferably between 0.2 and 1.0 mol/kg of catalyst.
When the reaction medium also comprises an alcohol, the amount of alkali metal or alkaline-earth metal hydroxide is greater than or equal to 0.05 mol/kg of catalyst, preferably between 0.1 and 10.0 mol/kg and even more preferably between 1.0 and 8.0 mol/kg.
The temperature at which the hemihydrogenation process is carried out is generally less than or equal to 150xc2x0 C., preferably less than or equal to 120xc2x0 C. and even more preferably less than or equal to 100xc2x0 C.
In practical terms, this temperature is usually between room temperature (about 20xc2x0 C.) and 100xc2x0 C.
Before, during or after heating, the reaction chamber is brought to a suitable hydrogen pressure, i.e., in practice, between 0.10 and 10 MPa.
The reaction time is variable depending on the reaction conditions and the catalyst.
In a batchwise operating mode, it can range from a few minutes to a few hours.
In a continuous operating mode, which is preferred for the process according to the invention, the reaction time is obviously not a fixable parameter.
It should be noted that a person skilled in the art can modify the chronology of the steps in the process according to the invention depending on the operating conditions. The order given above merely corresponds to one preferred, but not limiting, embodiment of the process according to the invention.
The other conditions which govern the hydrogenation (in continuous or batchwise mode) in accordance with the invention concern conventional technical arrangements which are known per se.
By means of all the advantageous arrangements mentioned above, the process of the invention makes it possible to hydrogenate dinitrile substrates into the corresponding aminonitriles, selectively, quickly, conveniently and economically.
The hemihydrogenation of adiponitrile gives 6-aminocapronitrile, a compound which can be readily converted by cyclizing hydrolysis into caprolactam, which is a starting material in the synthesis of nylon-6.
The invention is illustrated by the examples which follow, of the hemihydrogenation of adiponitrile into 6-aminocapronitrile.
In these examples, the following abbreviations may be used:
ADN=adiponitrile
ACN=aminocapronitrile
HMD=hexamethylenediamine
ACA=aminocaproamide
CVA=cyanovaleramide
DC=degree of conversion
RC=selectivity relative to the starting substrate converted (in this case relative to the ADN)
RY=yield relative to the starting substrate (ADN) used
IPOL=polarographic index reflecting the presence of impurities, in particular of imine type, which bring about in particular colorations and branching during polymerization of caprolactam (prepared by hydrolysis of ACN), if they are found in the latter material; this polarographic index is thus determined by polarography and is expressed as moles of imine function per metric ton of sample to be assayed.